ESR spin trapping is probably the only physical technique that can be used to detect thiyl radicals (RS ) formed under biological conditions. The spin trap of choice for detecting RS radical is usually 5,5-dimethyl-1-pyrroline N-oxide (DMPO). In general, the ESR spectra of DMPO/ SR adducts show subtle, but distinct, differences in their spectral pattern. However, under biological conditions, the spectra due to DMPO/ OH and DMPO/ SG (DMPO/glutathionyl adduct) are very similar at X-band (9.5 GHz), which makes data interpretation rather difficult. DMPO/ OH and DMPO/ SG adducts have slightly different g-values, and the spectral resolution of these adducts should, in theory, be increased at Q-band (35 GHz). However, the inherently low sensitivity of the conventional Varian Q-band spectrometer has precluded such applications. A recent breakthrough in Q-band instrumentation and methodology has increased the signal-to-noise ratio at Q-band by at least a factor of ten, thus opening new applications for Q-band ESR spectroscopy. In this study, we have compared the ESR spectra of spin adducts DMPO/ OH and DMPO/ SG formed from the oxidation of GSH by HO or ONOO using X-band (TE102) and Q-band (TE011 cavity). At Q-band, the spectral lines due to DMPO/ OH and DMPO/ SG were completely resolved. Using this approach, we have shown that DMPO/ OH adduct formed during oxidation of GSH by peroxynitrite is formed from the decomposition of DMPO/ OOH and not via trapping of HO . In addition, we have shown that the DMPO/ CO2 adduct is formed from the reaction between GS and formate.